Biological Effective Dose and Overall Treatment Time in the High Dose Rate Brachytherapy of Nasopharyngeal Carcinoma

Abstract

The aim of this study is to evaluate the decrease of biological effective dose (BED) and its correlation with local control of tumour in the treatment of nasopharyngeal carcinoma (NPC) when the overall treatment time is prolonged. A retrospective study was carried out on 39 NPC with stage II and III treated with fractionated High Dose Rate–Brachytherapy (HDR-BT)boost, following external beam radiation therapy (EBRT) treated in the period from 2009 to 2015. All patients were irradiated by HDR –Intra Luminal Radiotherapy (ILRT) following EBRT using a telecobalt unit and a technique that employed two lateral opposed fields with a dose of 66 ± 4 Gy. The total biological equivalent dose prescribed was 106.50 ± 9 Gy (range 92 - 123 Gy). The probabilities of disease recurrence within a median follow-up of 25 months (range 13 – 72 months) are expected as 0.03, 0.26, 0.58 and 0.90 (p=0.05) for the overall treatment time of 75, 150, 250 and 350 days respectively. It was observed that the local recurrence of disease increases with increased treatment time and it is significant (p=0.05) when the overall treatment time is above 100 days where BED lost becomes more than 0.10 Gy/day. The relative risk of local recurrence of stage III is about 2.59 times higher than that of stage II patients.